This invention relates generally to electronically commutated motors (ECM), and more specifically, to methods and systems for providing pulse width modulated (PWM) control signals to ECMs.
Power control systems for ECMs, also referred to as brushless direct current (DC) motors, may advantageously utilize pulse width modulation techniques for controlling motor operation. In general, such systems employ controllable power switching devices such as, for example, power transistors, silicon controlled rectifiers (SCR) or gate turn-off devices (GTO), serially connected between a power source and appropriate terminals of the motor. For a three phase motor, the system may utilize a three phase bridge arrangement with each of the three motor power terminals being connected to a corresponding leg of the three phase bridge. Each leg of the bridge may include a series connected pair of switching devices, one of the devices being operable to connect the motor terminal to a positive voltage source for supplying current to the motor and the other of the devices being operable to connect the motor terminal to a negative voltage source for allowing current to circulate out of the motor. Each switching device is responsive to a gating signal for becoming conductive and allowing current to pass in the associated winding phase of the motor. The gating signals are coupled to selected ones of the switching devices in an ECM control system in a manner to energize the windings of the motor in a predetermined sequence.
In a PWM system, either a current monitoring circuit or a voltage control circuit is effective to generate drive enable signals when motor current and/or voltage is less than a predetermined value. The drive enable signal, which is the PWM signal, and hereinafter referred to as the PWM enable signal, allows the gating signals to be coupled to the appropriate switching devices. Removal of the PWM enable signal inhibits coupling of the gating signals to the power switching devices.